1. Field of the Invention
The present invention relates to a sheet processing apparatus that discharges a sheet bundle comprised of a plurality of sheets.
2. Description of the Related Art
Conventionally, there has been known an image forming apparatus such as a copying machine, to which can be attached an original feeder that automatically feeds originals, and a sheet processing apparatus such as a finisher or a stitcher that selectively carries out a sorting process in which sheets with images recorded thereon are subjected to alignment, sorting, or the like, a stapling process in which a sheet bundle comprised of a plurality of sheets is selectively stapled, a folding process in which a sheet bundle comprised of a plurality of sheets is selectively folded, a stacking-process in which sheets or sheet bundles are stacked and stored, and other processes. The original feeder and the sheet processing apparatus are connected to each other to constitute the image forming apparatus.
An example of the stapling process carried out by the sheet processing apparatus is a one-point stapling in which a sheet bundle is stapled at one of two corners thereof in the direction of the sheet width. The stapled sheet bundle is discharged and stacked onto a tray by discharging rollers, which are driven with at a discharging speed controlled to a constant speed. The sheet processing apparatus is conventionally designed to be attached to a black-and-white image forming apparatus.
However, in the case where the sheet processing apparatus which has been attached to a black-and-white copying machine is attached as it is to a color copying machine, since the frictional coefficient of color output sheets is smaller than that of black-and-white output sheets, when a sheet bundle stapled at any corner thereof is discharged by a downstream discharging device disposed downstream in the sheet conveying direction while the discharge speed is controlled to a constant speed as in the prior art, the sheet bundle may be misaligned at a side edge thereof which has not been subjected to the stapling process, or may become torn or wrinkled in the vicinity of the stapling point. Such phenomena are caused by friction between sheets themselves or image toners recorded on the sheets, and if the friction between the sheets is small, significant misalignment of the sheets or the like occurs. For example, in the case of sheets (recording sheets) for use in black-and-white copying machines, no problem is encountered in the prior art control since the friction between the sheets (recording sheets) for use in black-and-white copying machines is great, but in the case of sheets (recording sheets) for use in color copying machines, where toner is applied over the entire surfaces of the sheets as in the case of a photographic image, the friction between the sheets tends to be small, and hence the misalignment of the sheets or the like is significant. Further, as the acceleration of the start-up speed at the start of conveyance of a sheet bundle increases, the friction between the sheets behaves as dynamic friction so that the friction coefficient between the sheets decreases (the coefficient of dynamic friction<the coefficient of static friction), and hence the sheets are more likely to be misaligned due to an impact occurring upon the start-up.